Energetic and dynamical instability of spin–orbit coupled Bose–Einstein condensate in a deep optical lattice

We investigate the energetic and dynamical instability of spin–orbit coupled Bose–Einstein condensate in a deep optical lattice via a tight-binding model. The stability phase diagram is completely revealed in full parameter space, while the dependence of superfluidity on the dispersion relation is illustrated explicitly. In the absence of spin–orbit coupling, the superfluidity only exists in the center of the Brillouin zone. However, the combination of spin–orbit coupling, Zeeman field, nonlinearity and optical lattice potential can modify the dispersion relation of the system, and change the position of Brillouin zone for generating the superfluidity. Thus, the superfluidity can appear in either the center or the other position of the Brillouin zone. Namely, in the center of the Brillouin zone, the system is either superfluid or Landau unstable, which depends on the momentum of the lowest energy. Therefore, the superfluidity can occur at optional position of the Brillouin zone by elaborating spin–orbit coupling, Zeeman splitting, nonlinearity and optical lattice potential. For the linear case, the system is always dynamically stable, however, the nonlinearity can induce the dynamical instability, and also expand the superfluid region. These predicted results can provide a theoretical evidence for exploring the superfluidity of the system experimentally.     

Superfluidity of “dirty” excitons

The effect of a random field due to impurities, boundary irregularities, and so on, on the superfluidity of a three-dimensional system of excitons and a quasi-two-dimensional system of direct or spatially indirect excitons is studied. The influence of a random field on the density of the superfluid component in the indicated excitonic systems at low temperatures T is investigated. The interaction between excitons is taken into account in the ladder approximation. For quasi-two-dimensional excitonic systems in a random field the Kosterlitz-Thouless temperature in the superfluid state is calculated.     

Impurity-induced phase transition in the interacting Bose gas: 1. Analytical results

We derive selfconsistency equations for the density relaxation and the longitudinal dynamical conductivity of the interacting Bose gas at temperature zero moving in a random potential. The equations describe a disorder-induced transition from a superfluid phase to an insulating phase, where the density is non-ergodic. The interaction of the bosons is treated in random phase approximation and the coupling to the impurities is calculated within generalized selfconsistent current relaxation theory. Scaling laws are discussed and explicit results are presented for the repulsive Bosgas with neutral impurities and for the charged Bose gas with charged impurities.     

Excitons and magnetoexcitons in coupled quantum nanostructures: the role of a dirty environment

The effect of a random field caused by impurities, interface roughness and so on, on the optical properties and superfluidity of a quasi-two-dimensional system of excitons is studied. The influence of a random field on the density of the superfluid component of excitonic systems at low temperatures is investigated. For quasi-two-dimensional excitonic systems in a random field the Kosterlitz–Thouless temperature in the superfluid state is calculated. The superfluidity and Bose–Einstein condensation of indirect excitons in coupled quantum dots are studied. Magnetoexciton light absorption in the disordered quantum wells is considered. The two-particle problem of the magnetoexciton motion in the external field depending on the external magnetic field is reduced to the one-particle motion with effective magnetic mass in some effective field. The energy and optical absorption of the magnetoexciton in a single and coupled quantum dots are studied using the effective-magnetic-mass Hamiltonian. In the coherent potential approximation the coefficient of magnetoexciton optical absorption in single and coupled quantum wells is calculated. In the strong magnetic fields the exciton peak decreases with magnetic field increasing in accordance with the experimental data. The localization of direct and indirect magnetoexcitons is investigated. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Dissipationless transport in low-density bilayer systems

In a bilayer electronic system the layer index may be viewed as the z component of an isospin- 1 / 2. An XY isospin-ordered ferromagnetic phase was observed in quantum Hall systems and is predicted to exist at zero magnetic field at low density. This phase is a superfluid for opposite currents in the two layers. At B = 0 the system is gapless but superfluidity is not destroyed by weak disorder. In the quantum Hall case, weak disorder generates a random gauge field which probably does not destroy superfluidity. Experimental signatures include Coulomb drag and collective mode measurements.     

Superfluidity versus disorder in the discrete nonlinear Schrödinger equation

We study the discrete nonlinear Schr?dinger equation (DNLS) in an annular geometry with on-site defects. The dynamics of a traveling plane-wave maps onto an effective nonrigid pendulum Hamiltonian. The different regimes include the complete reflection and refocusing of the initial wave, solitonic structures, and a superfluid state. In the superfluid regime, which occurs above a critical value of nonlinearity, a plane wave travels coherently through the randomly distributed defects. This superfluidity criterion for the DNLS is analogous to (yet very different from) the Landau superfluidity criteria in translationally invariant systems. Experimental implications for the physics of Bose-Einstein condensate gases trapped in optical potentials and of arrays of optical fibers are discussed.     

Transition to superfluidity in strongly coupled fermion systems: low density limit

The rôle of interacting bound pairs in strongly coupled fermion systems is considered in connection with the transition to superfluidity. Model calculations are performed for finite-range separable interaction potentials. The results of a cluster-Hartree-Fock approximation are compared with recent approaches improving the Nozières and Schmitt-Rink theory. In the low-density strong coupling limit, a first order transition to the superfluid phase is obtained.     

Dynamical theory of superfluidity in one dimension

A theory accounting for the dynamical aspects of the superfluid response of one dimensional (1D) quantum fluids is reported. In long 1D systems, the onset of superfluidity is related to the dynamical suppression of quantum phase slips at low temperatures. The effect of this suppression as a function of frequency and temperature is discussed within the framework of the experimentally relevant momentum response function. Applications of these results to the understanding of the superfluid properties of helium confined in 1D pores with nanometer diameter, dislocations in solid 4He, and ultracold atomic gases are also briefly discussed.     

When superfluids are a drag

The article considers the dramatic phenomenon of seemingly frictionless flow of slow-moving superfluids. Specifically the question of whether an object in a superfluid flow experiences any drag force is addressed. A brief account is given of the history of this problem and it is argued that recent advances in ultracold atomic physics can shed much new light on this problem. The article presents the commonly held notion that sufficiently slow-moving superfluids can flow without drag and also discusses research suggesting that scattering quantum fluctuations might cause drag in a superfluid moving at any speed.     

Variational solution of a superfluidity model

We derive, using the variational principle for the free energy, the solution of a model for superfluidity proposed in a previous paper. The solution has the phase diagram and the excitation spectrum expected from a superfluid. We draw the attention to the fact that the model is obtained, as compared with the Bogoliubov model, by a milder suppressing of interactions with the zero mode fluctuations, leading to the good spectral properties of superfluidity.     

Effects of Thermally Induced Roton-Like Excitation on the Superfluid Density of a Quasi-2D Dipolar Bose Condensed Gas

Using the Green's function approach, the density–density correlation function and the dielectric function in the random-phase approximation for a quasi-two-dimensional (quasi-2D) dipolar Bose gas are derived. From the pole of the density correlation function, by considering thermally induced roton-like excitations, the excitation spectrum of the system is calculated. It is shown that the position and depth of the roton minimum of the excitation spectrum are tunable by changing the temperature. To show how the position of the roton minimum influences the phenomenon of superfluidity, the superfluid density of the system is obtained and it is shown that the interplay of the thermal rotonization, contact and dipole–dipole interaction (DDI) can affect the superfluid fraction of a quasi-2D Bose gas. It is found that contact, DDI interactions, and thermally induced rotons enhance the fluctuations and reduce the superfluid density. In the absence of DDI and thermally induced rotons, the usual T³ dependence of superfluid density in 2D is obtained and the correction T⁴ term arises from DDI. It is shown that if the roton minimum is close to zero, the thermally induced rotons change the linear temperature dependence of the superfluid fraction, leading to a transition to nontrivial supersolid phase.     

Mott-Hubbard transition of bosons in optical lattices with two-body interactions

In this paper, based on the Bose-Hubbard model with two-body on-site interactions, we study the quantum phase transition between the superfluid state and the Mott-insulator state. With the decoupling approximation, we get the relation between the weak superfluidity and dimensionless chemical potential with different particle number and different dimensionless interaction strength, and the relation between the weak superfluidity and the reciprocal of dimensionless interaction strength with different particle number. We also calculate the corresponding experimental parameters.     

Phase diagram for quantum hall bilayers at nu=1

We present a phase diagram for a double quantum well bilayer electron gas in the quantum Hall regime at a total filling factor nu=1, based on exact numerical calculations of the topological Chern number matrix and the (interlayer) superfluid density. We find three phases: a quantized Hall state with pseudospin superfluidity, a quantized Hall state with pseudospin "gauge-glass" order, and a decoupled composite Fermi liquid. Comparison with experiments provides a consistent explanation of the observed quantum Hall plateau, Hall drag plateau, and vanishing Hall drag resistance, as well as the zero-bias conductance peak effect, and suggests some interesting points to pursue experimentally.     

Mott-Hubbard transition of bosons in optical lattices with two-body interactions

In this paper, based on the Bose-Hubbard model with two-body on-site interactions, we study the quantum phase transition between the superfluid state and the Mott-insulator state. With the decoupling approximation, we get the relation between the weak superfluidity and dimensionless chemical potential with different particle number and different dimensionless interaction strength, and the relation between the weak superfluidity and the reciprocal of dimensionless interaction strength with different particle number. We also calculate the corresponding experimental parameters.     

Superfluidity near phase separation in Bose-Fermi mixtures

We study the transition to fermion pair superfluidity in a mixture of interacting bosonic and fermionic atoms. The fermion interaction induced by the bosons and the dynamical screening of the condensate phonons due to fermions are included using the nonperturbative Hamiltonian flow equations. We determine the bosonic spectrum near the transition towards phase separation and find that the superfluid transition temperature may be increased substantially due to phonon damping.     

Intermediate-temperature superfluidity in an atomic fermi gas with population imbalance

We derive the underlying finite temperature theory which describes Fermi gas superfluidity with population imbalance in a homogeneous system. We compute the pair formation temperature, superfluid transition temperature Tc, and superfluid density in a manner consistent with the standard ground state equations and, thereby, present a complete phase diagram. Finite temperature stabilizes superfluidity, as manifested by two solutions for Tc or by low T instabilities. At unitarity, the polarized state is an "intermediate-temperature superfluid."     

Anomalous suppression of superfluidity in 4He confined in a nanoporous glass

We explore the superfluidity of 4He confined in a porous glass, which has nanopores of 2.5 nm in diameter, at pressures up to 5 MPa. With increasing pressure, the superfluidity is drastically suppressed, and the superfluid transition temperature approaches 0 K at some critical pressure, Pc approximately 3.4 MPa. The feature suggests that the extreme confinement of 4He into the nanopores induces a quantum phase transition from a superfluid to a nonsuperfluid at 0 K and at Pc.     

Anisotropic Superfluidity in a Weakly Interacting Condensate of Quasi‐2D Photons

The anisotropic superfluidity in a weakly interacting two‐dimensional Bose gas of photons in a dye‐filled optical microcavity is investigated, taking into account the dependence of the photon effective mass on the in‐plane coordinate. With the use of the generalized Gross–Pitaevskii equation and the Bogoliubov approach, it is shown that the modulation of the microcavity width leads to an effective periodic potential and the periodicity of the condensate wave function, and both the condensate energy and the spectrum of elementary excitations depend on the direction of motion. The anisotropic character of the dynamical and superfluid properties, such as helicity modulus, superfluid density, and sound velocity, as well as experimentally observable manifestations of their anisotropy are described.